1. Field of the Invention
The present invention relates to an apparatus for mutual conversion between circular motion and reciprocal motion which is used for converting reciprocal motion of, e.g., the piston of a four-cycle reciprocating engine into rotary motion of a crankshaft.
2. Description of the Related Art
FIG. 24 shows a cylinder section of a conventional four-cycle reciprocating engine. This engine has a structure in which a connecting rod 4 is connected between a piston 2 and a crankshaft 3, in order to convert reciprocal motion of the piston 2 moving up and down in the cylinder 1, into rotary motion of the crankshaft 3. In this figure, the reference numeral 5 denotes a radiating plate.
A known factor that hinders an increase in output efficiency of this kind of four-cycle reciprocating engine is a side thrust unnecessarily generated by the piston 2. This side thrust cannot be avoided since the piston 2 and the crankshaft 3 are coupled to each other by using the connecting rod 4. More specifically, since a friction heat is generated by the side thrust during process of the reciprocal motion of the piston 2, all the mechanical energy cannot be transmitted from the piston 2 to the crankshaft 3. The side thrust is also a factor which vibrates and shocks the piston 2.
Thus, conventionally, the connecting rod intervenes linear motion into rotary motion. However, since the piston oscillates as the crank moves, a side thrust is generated between the piston and the cylinder, e.g., in a four-cycle engine. Due to an energy loss thus caused by the side thrust, the engine idling speed must be increased to about 1,000 rpm, causing a problem in fuel consumption.
The side thrust causes not only the energy loss but also cracking and breaking of a piston through collision of the piston against side walls of the cylinder. In order to prevent these damages, the piston must be made of a heavy, strong metal, and as a result, the weight of the piston cannot be decreased by making the piston with, e.g., ceramics.
FIG. 5 shows a relationship between a piston position in a cylinder and an engine rotation angle in a conventional four-cycle reciprocating engine. In FIG. 5, a continuous line graphically shows an ideal displacement of a piston in a cylinder. Compared with this displacement, a piston of a conventional reciprocating engine moves as indicated by a broken line in the figure and shows that a pressure raising speed of a fuel gas is later than an ideal speed within a compression process from 0.degree. to 180.degree. while the lowering of the pressure of the fuel gas is earlier than the ideal speed within a process from 180.degree. to 360.degree.. For example, where ignition is obtained at a position of 160.degree., the compression ratio of a fuel gas of a conventional engine is smaller than the ideal compression ratio (which is called a late rise of a piston), and therefore, an expansion pressure is consequently decreased. Further, in the expansion process, the pressure of the combustion gas decreases earlier than the ideal pressure (which is called an early fall of the fuel piston), and the pressure generated by combustion of the fuel gas cannot sufficiently be converted into a mechanical energy.
FIG. 6 is a graph showing a relationship as a conversion efficiency between a gas volume V (m.sup.2) in a cylinder and a gas pressure MPa (in mega-pascal) in case where a combustion energy is converted into a mechanical energy. In this figure, a broken line indicates an energy efficiency of a conventional reciprocating engine and a solid line represents that of the present invention.
The late rise and early fall of a piston which reduce a heat efficiency of a reciprocating engine are called subtraction motion. Particularly, in engines for ships, the connecting rod is designed to be as long as possible in order to eliminate the subtraction motion and as a result, these engines sometimes have a height of 15 m.
FIG. 7 is a view analyzing operations of the piston 2, connecting rod 4, and the crankshaft 3 of FIG. 24, where s denotes the process of the piston 2, L denotes the length of the connecting rod 4, r denotes the rotation radius of the crankshaft 3, .alpha. denotes an angle between the connecting rod 4 and a line connecting centers of the piston 2 and the crankshaft 3 with each other, and .theta. denotes a rotation angle of the crankshaft 3.
The displacement of a piston of a conventional engine is represented by the following equation. EQU s=r(1-cos .theta.)+L(1-cos .alpha.) L.multidot.sin .alpha.=r.multidot.sin .theta.
This equation is developed as follows: EQU s=r(1-cos .theta.)+L(1-(1-r**2 sin**2.theta./L**2)**0.5) (1)
where **2 denotes a square and **0.5 denotes a square root.
As can be seen from the equation (1), the displacement s of the piston includes a term of the 0.5th degrees in the rotation angle .theta. of the crankshaft 3. Therefore, the displacement s of the piston does not show a shape of an ideal sine wave.
Further, a conventional engine uses a flywheel and counter-weight for a crankshaft to smoothen engine rotation. These components, however, absorb an energy generated by the engine during engine acceleration, and the energy thus absorbed is then consumed as a wasteful thermal energy during braking for engine deceleration.